1. Field of the Invention
The present invention relates to improvements in apparatus for reliably aspirating small aliquots (in the microliter range) of liquid sample from a vessel, e.g. a test tube or vial. The liquid aspirating apparatus of the invention is particularly useful in biological and chemical instruments that are required to automatically withdraw small, yet precise, volumes of liquid from a test tube or vial for subsequent use or analysis.
2. Discussion of the Prior Art
Liquid transport systems are known for aspirating small volumes of a liquid sample from a vessel at one location and for dispensing the aspirated sample into a reaction chamber at another location. Such systems are used, for example, in hematology instruments adapted to automatically analyze a blood sample. In such instruments, the tip of an aspiration probe or needle is lowered into a vial containing a blood sample to a level below that of the sample in the vial. A negative pressure (vacuum) is then applied to the probe to draw all or a portion of the sample into and through the probe to a location where the aspirated sample can be segmented into smaller and precise volumes for analysis.
To aspirate relatively minute volumes of liquid samples contained in a vial, or to assure that the entire contents of a liquid-containing vial are extracted by an aspirating system of the type described above, it is necessary that the tip of the aspirating probe contacts the inside bottom of the vial during the aspiration process. In those systems in which the vials and aspirating probes are of standard size, virtual contact between the probe tip and the vial bottom can be assured by precisely controlling both the position of the vial and the vertical displacement of the aspirating probe. However, in those systems in which the vial size (length) is variable, achieving the desired contact can be problematic. Various schemes have been devised for detecting that the tip of the aspirating probe is always immersed in the liquid during aspiration; see, for example, the disclosure of the commonly assigned U.S. Pat. No. 4,326,851 to Bello et al. In all these schemes it is common to arrest downward movement of the aspirating probe before the probe tip contacts the bottom of the vial for fear of breaking the vial; as a result, small volumes of sample in the vial cannot be removed, and/or the last drop of a relatively large volume of liquid cannot be aspirated.
In view of the foregoing discussion, an object of this invention is to provide an improved liquid aspiration apparatus of the type described, one that is improved from the standpoint that the last drop of liquid in a vessel can be reliably aspirated.
According to the invention, a conventional liquid aspiration apparatus is modified to include a mechanism for determining that the tip of a liquid-aspiration probe is continuously maintained in engagement with the inside bottom of a liquid-containing vessel during aspiration of the liquid therein. Such a bottom-sensing mechanism is adapted to sense a predetermined forcible interaction between the probe tip and the vessel bottom. In a preferred embodiment, such forcible interaction is sensed by detecting movement of a movably mounted platform that supports the vessel during aspiration. In a second embodiment, the forcible interaction is sensed by detecting the back-emf in a motor winding used to advance the aspiration probe.
In accordance with the preferred embodiment of the invention, the improved liquid aspirating apparatus of the invention comprises:
(a) a platform for supporting a liquid-containing vessel in an upright orientation so that a longitudinal axis of the vessel is substantially vertical, such platform being movably mounted on a frame for vertical movement relative to a nominal position;
(b) spring means for biasing the platform towards its nominal position;
(c) aspirating means including an aspirating probe movable along the longitudinal axis of the vessel supported by the movably-mounted platform, such probe having a tip through which liquid can be aspirated from the vessel;
(d) drive means for selectively advancing the aspirating probe along the vessel""s longitudinal axis to cause the probe tip to engage the inside bottom of the vessel and to exert a downward force on the vessel, thereby causing the platform to move away from its nominal position against the force exerted by the spring means;
(e) movement-detecting means for detecting a predetermined movement of the platform relative to its nominal position to sense that the probe tip has been sufficiently advanced to forcibly engage the inside bottom of the vessel, such movement detecting means producing a signal responsive to such movement detection; and
(f) circuit means responsive to the signal for controlling the drive means to maintain such forcible engagement during liquid aspiration.
According to the alternative embodiment, the above-noted drive means comprises an electric motor having a motor winding, and movement-detecting means is replaced with a circuit for detecting the back electromotive force in the motor winding as caused by the resistance of the vessel bottom to further movement of the aspirating probe after initial contact is made between the probe tip and the vessel bottom.
The invention and its various advantages will become apparent to those skilled in the art from the ensuing description of a preferred embodiment, reference being made to the accompanying drawings.